In a conductor tracer of the type disclosed in U.S. Pat. No. 4,491,785, a transmitter draws or absorbs a current tracing signal from a conductor energized by an AC power source. The current tracing signal creates a predetermined magnetic field tracing signal around the conductor. The magnetic field tracing signal has a strength characteristic which is directly related to the magnitude of the current tracing signal, and has polarity characteristic which is directly related to the direction of current flow in the conductor. A receiver of the conductor tracer detects the magnetic field tracing signal, and a display of the magnitude of the detected signal is used to distinguish the conductor carrying the current tracing signal from adjacent conductors which are not carrying the current tracing signal.
The magnetic field tracing signal is influenced by a number of factors. The relative voltage between the conductor and ground is constantly changing, and there is a tendency for the amount of current of the current tracing signal to correspondingly fluctuate. By maintaining the current tracing signal at a relatively constant magnitude independent of the voltage fluctuations, a relatively constant magnitude magnetic field tracing signal is created about the conductor. The relatively constant magnetic field tracing signal makes it easier to distinguish the conductor carrying the tracing signal from adjacent conductors. Approaches to regulating the amount of current drawn by the transmitter to create a generally uniform strength magnetic field tracing signal are disclosed in U.S. Pat. Nos. 4,491,785 and 4,642,556, and in the above mentioned application for Conductor Tracer with Improved Current Regulating Transmitter.
The relative polarity and current flow direction also changes with each successive half cycle of the AC energizing power. Due to the manner in which the current tracing signals have previously been applied to the conductor, the polarity of the magnetic field tracing signal reverses with each successive half cycle of AC energizing power. The constantly reversing polarity of the magnetic field tracing signal detracts from the ability of prior receivers to detect the magnitude of the magnetic field tracing signal. The prior art magnetic field tracing signal effects and the prior art conductor tracer transmitters and receivers are better understood by reference to FIGS. 1, 2A, 2B, 2C, 2D, 3, 4A and 4B.
Referring to FIG. 1, a simplified prior art conductor tracer transmitter 10 is connected through a power plug 12 to the conductor (not shown) which is to be traced and identified. The conductor is energized by the conventional AC sine wave power signal, which is shown in FIG. 2A. A full wave bridge rectifier 14 of the transmitter 10, shown in FIG. 1, rectifies the AC energizing signal to the full wave rectified form shown in FIG. 2B. A conventional gated oscillator 16 creates a series of trigger pulses which are applied to a power transistor 18. The transistor 18 is connected in an emitter follower arrangement through the resistor 20 connected to its emitter. The trigger pulses from the gated oscillator 16 occur at a predetermined high frequency and are gated on and off in a conventional duty cycle fashion. The transistor 18 conducts current pulses of approximately constant current from the bridge rectifier 14 through the resistor 20 in response to each constant magnitude trigger pulse from the oscillator 16.
The current pulses conducted from the rectified AC energizing signal are shown in FIG. 2C. All the pulses shown in FIG. 2C are of constant width and would be of constant height, and therefore constant energy content, except that slightly before and after the zero crossing points of the AC power signal and the rectified AC signal shown in FIGS. 2A and 2B respectively, an insufficient voltage exists on the conductor to conduct constant energy pulses. The pulses shown in FIG. 2C, are conducted from the conductor by the bridge rectifier 14 (FIG. 1) and appear on the conductor in the form of the current tracing signal shown in FIG. 2D. Stated another way, the current flow reverses on the conductor in successive half cycles, and FIG. 2D shows that the current pulses shown in FIG. 2C flow in opposite directions on the conductor during the positive and negative half cycles of the AC energizing signal. The magnetic field tracing signal about the conductor has one polarity during the positive half cycles of the AC power signal and has the opposite polarity during the negative half cycles of the AC power signal, because the direction of current flow on the conductor reverses.
The interval, time width, spacing and frequency of the pulses of the current tracing signal remain consistent during successive half cycles of the applied AC energizing signal, because such characteristics are established by the gated oscillator. Even though the characteristics of these pulses remain consistent, the reversal in the direction of current flow over the conductor during successive half cycles of the AC power signal has the effect of phase shifting the frequency component of the magnetic field and current tracing signals by 180.degree. due to the current flow reversals.
The 180.degree. phase shift during successive half cycles of the AC power creates difficulties in the detection of the magnetic field tracing signal as will be described in reference to the simplified conductor tracer receiver shown in FIG. 3. The receiver 22 includes a resonant circuit 24 which includes an inductor 26 and a capacitor 28. The components 26 and 28 are selected to establish their resonant frequency at approximately the same frequency of the high frequency component of the magnetic field and current tracing signals. The magnetic field tracing signal induces a signal in the inductor 26 when the receiver is brought into physical proximity with the conductor carrying the current tracing signal. The resonant circuit 24 starts ringing at its resonant frequency as is shown in FIG. 4A. The frequency is passed through a band pass filter 30 whose band pass frequency is also that frequency of the high frequency component of the tracing signal. The envelope of the signals passed through the filter is shown in FIG. 4B. A detector 32 of the receiver 22 rectifies the high frequency signal from the filter 30 and develops a signal representative of its magnitude during the on time of the duty cycle of the tracing signal. The magnitude of the detector signal is displayed at a display 34 of the receiver 22.
Once the resonant circuit 24 starts ringing, each cycle of the high frequency magnetic field tracing signal reinforces the ringing effect and the magnitude of the oscillating signal in the resonant circuit 24 builds. With the next successive half cycle of the AC energizing signal, the polarity of the magnetic field tracing signal reverses, and its high frequency component becomes 180.degree. out of phase relative to the phase of the signal oscillating in the resonant circuit. As a consequence, the ringing in the resonant circuit 24 is no longer reinforced, but is quickly cancelled by the 180.degree. phase reversal. The cancelling effect occurs rapidly after the polarity reversal, but nonetheless takes a finite amount of time. The effect is shown in FIG. 4A where the ringing frequency from the previous half cycle terminates and the ringing starts at the same frequency but phase shifted by 180.degree. at the beginning of each successive half cycle of the AC energizing signal.
The effect of the phase reversal in the receiver 22 is illustrated in FIG. 4B. As soon as the phase reversal occurs, the signal in the resonant circuit 24, filter 30 and detector 32 must first collapse, and thereafter build up again in the resonant circuit in response to the phase reversal of the high frequency component of the magnetic field tracing signal. During this collapse-and-buildup time period, the display 34 provides an uncertain indication, the filter 30 must respond to the new phase relationship, and the detector 32 must dissipate at least a portion of its prior level and establish a new level. The effect of the phase reversals is therefore to inhibit the ability of the receiver, and the user, to distinguish between adjacent conductors, because a constant polarity magnetic field tracing signal is not present on the conductor.
One solution to the prior problems of phase reversals in conductor tracers is described in the U.S. patent application for a Conductor Tracer Receiver with Immunity Phase Reversals in the Transmitter Signal. Another solution to these problems is described below.